1. Field of the Invention
The present invention relates to a lens driving device which causes a moving lens in a lens barrel provided in a zoom system or a focus system to freely move forward and backward along the optical axis in an image pickup apparatus such as a video camera or a still camera.
2. Description of the Related Art
In recent years, with miniaturization of image pickup apparatuses such as video cameras and still cameras, demand for miniaturizing lens barrels has been growing. Accordingly, there has been an increasing demand for reducing the number of components disposed within lens barrels and miniaturizing such components.
Also, with miniaturization of lens barrels, photographic optical systems have decreased markedly in size. Miniaturizing image pickup apparatuses and shortening the movement distances of moving lenses have greatly contributed to the miniaturization of photographic optical systems.
If, however, the optical sensitivities of moving lenses are increased, subjects may fall out of focus to a large extent or zoom magnifications may vary in response to even slight changes in the positions of the moving lenses, which would have serious affects on captured images. Thus, improving positional accuracies of moving lenses is essential to further miniaturization of image pickup apparatuses by increasing the optical sensitivities of moving lenses.
Where conventional lens barrels are concerned, a method has been proposed in which a lens holder is supported by a thin shaft called a rod or a guide bar, and a lead screw engaged with the lens holder is rotatively driven by a motor such as a pulse motor so as to cause the lens holder to move along the optical axis.
For example, lens driving devices in which two guide bars for guiding a lens holder holding a group of moving lenses (one of the two guide bars performs the function of inhibiting rotation of the lens holder about the guide bar), a rack member mounted on the lens holder is engaged with a motor-driven lead screw to rotate the lead screw so as to cause the lens holder guided by the two guide bars to move along the optical axis have conventionally been in widespread use (see Japanese Laid-Open Patent Publication (Kokai) No. 2004-94172, for example).
There have been other conventional lens driving devices in which a lead screw having not only its original function of transferring drive power to a lens holder but also a function of guiding a lens holder along the optical axis and positioning the lens holder in a plane orthogonal to the optical axis, that is, a guide shaft functioning as a guide bar and a lead screw is used, so that only one guide bar can perform the functions of guiding and positioning although two guide bars had been needed in the past (see Japanese Laid-Open Patent Publication (Kokai) No. H10-20177, for example).
There have been proposed still other lens driving devices in which a lens holder is fixed to a guide shaft functioning as a guide bar and a guide screw, that is, a guide shaft having a guide portion and a male thread portion formed coaxially with each other, a female thread portion is formed in the bore portion of the rotor of a stepping motor, and the motor is disposed concentrically with the guide shaft (see Japanese Laid-Open Patent Publication (Kokai) No. H06-250070, for example).
The above-mentioned conventional lens driving devices, however, needs to be equipped with the two guide bars as well as the lead screw, and a member for integrally fixing the motor and the lead screw, and therefore have the problem that a large number of components are needed, which inhibits reduction in the size and weight of the devices.
In these conventional lens driving devices, the rack member that receives drive power from the lead screw is disposed on the outer side of the two guide bars as viewed in the direction of the lens diameter, and therefore, there may be cases where there is a long distance between the moving lens and the rack member. In such cases, the rack member tends to collapse along the optical axis during transfer of drive power from the lead screw to the rack member, degrading the positional accuracy of the moving lens along the optical axis. Further, the engagement of the rack member with the lead screw tends to cause so-called tooth skip due to external forces or vibrations, and it is therefore difficult to control the lens position with accuracy.
In the above-mentioned other conventional lens driving devices, since one guide shaft functions as a lead screw and a guide bar, a low component count can be realized to reduce the size and weight of the devices. In these conventional lens driving devices, however, the guide shaft itself is caused to rotate by a motor. In addition, components shaped like a guide shaft always have a warp in terms of machining accuracy. Thus, when the guide shaft is caused to rotate by the motor, a vibration of at least several μm occurs in a plane perpendicular to the axis of the guide shaft, and accordingly, the lens holder supported by the guide shaft also vibrates with each turn of the lead screw. As a consequence, image shake occurs to cause image degradation.
In general, in driving lenses of a camera or the like, a plurality of lens holders are caused to move by different amounts within a unit time in many cases. In these conventional lens driving devices, however, the guide shaft itself rotates, and hence only one lens holder can be caused to move with respect to one guide shaft. That is, in the case of causing a plurality of lens holders to move by different amounts, it is necessary to provide a guide shaft for each lens holder, which inhibits space saving.
In the still other conventional lens driving devices mentioned above, since one guide shaft functions as a lead screw and a guide bar, a low component count can be realized to reduce the size and weight of the devices. Further, in these conventional lens driving devices, since the guide shaft is not rotated integrally with the lens holder, image shake resulting from rotation of the guide shaft does not occur when the lens holder is caused to move, as distinct from the other conventional lens driving devices mentioned above.
In these conventional lens driving devices, however, since the guide shaft and the lens holder are configured as one integral unit, only one lens holder can be caused to move with respect to one guide shaft. That is, in the case of causing a plurality of lens holders to move by different amounts, it is necessary to provide a guide shaft for each lens holder, which inhibits space saving.
Also, in these conventional lens driving devices, since the guide shaft and the lens holder are configured as one integral unit, the guide shaft projects along the optical axis, i.e. forward and rearward of the lens barrel in response to movement of the lens. For this reason, in the case of causing the lens to move a relatively long distance (for example, about 10 mm) along the optical axis, it is necessary to provide a space for evading the projecting guide shaft.
In general, electrical components such as a shutter unit are mounted on a lens holder in many cases. In these conventional lens driving devices, the motor is disposed in the vicinity of the lens holder, but the motor and the lens holder are not configured as an integral unit. For this reason, in the case where a shutter unit or the like is mounted on the lens holder, the distance between the motor and the shutter unit varies during movement of the lens. To cope with this, it is necessary to provide wiring of two systems for supplying power or to bifurcate wiring for supplying power.